Method for producing castings, molding sand and its use for carrying out said method

ABSTRACT

The invention provides a method with which high-quality complex-shaped castings can be manufactured and with which, after the end of the casting process, the casting mould parts can be easily removed out of or from the casting without any risk. To achieve this the following steps are carried out:  
     Manufacture of a mould material by mixing of a basic mould material, inert in comparison with the molten metal and pourable, which under heating expands less than quartz sand, and a binding agent, which under heating expands differently in comparison with the basic mould material, the manufacture of a casting mould part from the mould material, assembly of a casting mould making use of the casting mould part, pouring of the molten metal into the casting mould to form a casting, cooling of the casting over a solidification and cooling period, in which the casting mould part disintegrates self-actingly into fragments, removal of the fragments of the casting mould part from or out of the casting, and reprocessing of the fragments of the mould material to form a pourable basic mould material.

[0001] The invention relates to a method for the manufacture of castingsfrom a molten metal, in particular molten light metal, such as moltenaluminium.

[0002] In addition to this, the invention relates to a moulding materialand its use for the manufacture of casting mould parts, which are usedfor the casting of molten metal, in particular molten light metal, suchas molten aluminium. Such casting mould parts may, for example, becasting cores, through which cavities are formed in the interior of thecastings to be produced. Likewise, the casting mould parts according tothe invention can be structural elements from which a multi-part castingmould can be composed, by means of which the external shape of thecasting to be manufactured is determined.

[0003] The manufacture of structural components made of metal by castingtechnology requires casting mould parts by means of which, on the onehand, the inner shape and, on the other, the outer shape of theworkpieces which are to be cast are determined. Such casting mould partscan accordingly be casting cores, by means of which cavities are formedin the interior of the casting which is to be created, or casting mouldelements from which a multi-part casting mould is assembled, whichdetermines the outer shape of the casting which is to be created.

[0004] To manufacture casting mould parts, mould material systems areused, as a rule, which are composed of a basic mould material and abinding agent. These two components are mixed with each other, shaped,and processed in a suitable hardening process to form a compact body. Asa basic mould material, quartz sand is usually used, which in themajority of applications is bonded with an organic binding agent.

[0005] The use of quartz sand as a basic material for the manufacture ofcasting mould parts has proved its value in many respects in particularin the sector of the casting of light metal materials. Thus, such quartzsand can be obtained economically and is characterised by easyprocessing qualities and good quality in the imaging of the mouldelements of the casting which is to be produced in each case.

[0006] As a more environmentally compatible alternative to organicbinding agents, the use of binding agents on a water glass base has beenproposed. This water glass binding agent is mixed with the mould sand.The mixture obtained is then shot into the moulding box of a mouldingmachine, in which a cavity is formed which represents the shape of themould part which is to be produced. Next, by supplying heat, water isdrawn off from the mixture introduced into the mould. The supply of heatcan in this situation be effected by an appropriate heating of themoulding box or by means of microwave heating taking effect directly onthe mixture (WO-A-86/00033, EP 0 917 499 B1, DE 196 32 293 A1).

[0007] In order to guarantee an optimum operational result when castingthe molten metal, the mould material used for the manufacture of thecasting mould parts must have high strength and high dimensionalconsistency, which will still be maintained under the loads incurredduring the manufacture of the casting mould and the casting of the melt.In addition to this, the moulding material should be easy to removeafter the casting. This becomes particularly important if casting coresare used which form complex shaped interiors in the casting.

[0008] Finally, the mould materials should be capable of beingregenerated after use in such a way that the highest possible quota ofreuse can be achieved with the basic mould material. This can beachieved in a known manner by the use of inorganic binding agents, whichrelease low emissions during the manufacture of the mould parts and,after the end of the casting process, can be incinerated almost free ofany residue under the effect of sufficiently high temperatures.

[0009] In practical application it has been shown that the known mouldmaterial systems, regardless of whether they contain organic orinorganic binding agents, under normal conditions have the propertiesrequired for optimum operational result.

[0010] However, especially with thin-walled mould parts, such as areused, for example, in the casting of engine blocks or cylinder heads, asmould cores for oil channels, as a result of unavoidable thermalexpansion the requirements for the dimensional stability of the castingcan no longer be fulfilled.

[0011] A further problem with the casting of complexly shaped castingsmaking use of casting mould parts manufactured by conventional meanslies in the fact that the sand can only be removed from the casting withdifficulty after cooling. To do this, the casting is usually subjectedto shaking or knocking, which causes the disintegration of the castingcores on the inside of the casting and of the mould parts adhering tothe outside of the casting, and is intended to promote the pouring outof the mould material particles obtained. These mechanical methods forthe removal of the mould parts do, however, incur the risk of damage tothe casting; for example, especially with filigree-shaped or thin-walledcomponents, crack formation may occur.

[0012] It has therefore been proposed that, instead of mechanicalmeasures taking effect on the casting, the casting should be heatedsufficiently for the binding agent to be combusted until the basic mouldmaterial alone remain, and can then be easily removed out of and fromthe casting as pourable material. The expenditure required in terms ofapparatus for this is considerable. In addition to this, thetemperatures required for the combustion of the binding agent are sohigh that the heating will inevitably also cause a change in theproperties of the metallic castings.

[0013] The object of the invention was to provide a method with whichhigh-quality complex-shaped castings can be manufactured, and withwhich, after the end of the casting process, the casting mould parts canalso be removed out of and from the casting in a simple manner andwithout any risk. In addition to this, a mould material is also intendedto be provided with which mould parts can be manufactured which aresuitable for the production of high-quality complex-shaped castings, andwhich, after the termination of the casting process, can be removed outof and from the casting in a simple manner and without any risk.

[0014] This object is solved in respect of the method according to theinvention by a method for the manufacture of castings from molten metal,in particular molten light metal, in which the following steps arecarried out:

[0015] Manufacture of a casting mould part from the mould material,which is mixed from a pourable basic mould material which is inert inrelation to the molten metal and from a binding agent, wherein thethermal expansion behaviour of basic mould material and binding agentare adjusted to one another in such a way that the coefficient ofthermal expansion of the molten metal lies above the coefficient ofthermal expansion of the casting mould part manufactured from the mouldmaterial,

[0016] Assembling a casting mould making use of the casting mould part,

[0017] Pouring of the molten metal into the casting mould to form acasting,

[0018] Cooling of the casting over a solidification and cooling periodin which the casting mould part self actively disintegrates intofragments,

[0019] Removal of the fragments of the casting mould part from or out ofthe casting,

[0020] Reprocessing of the fragments of the mould material to formpourable basic mould material.

[0021] On the other hand, the object described above is also solved by amould material for the manufacture of casting mould parts for thecasting of molten metal, in particular of molten light metal, consistingof a pourable mixture of basic mould material, inert in relation to themolten metal, and a binding agent mixed with the basic mould material,wherein the thermal expansion behaviour of the basic mould material andbinding agent are adjusted to one another in such a way that thecoefficient of thermal expansion of the molten metal lies above thecoefficient of thermal expansion of the casting mould part manufacturedfrom the mould material.

[0022] The invention is based on the understanding that, by theselection of a suitable mould material, casting mould parts can beproduced which combine in an optimum manner the properties required forthe simple, reliable, and environmentally-friendly manufacture ofhigh-quality and dimensionally-stable castings.

[0023] Mould material according to the invention provides an optimumcombination of the properties which are a precondition for themanufacture of a high-quality casting with, at the same time, a simpleway of manufacture. To this purpose, the mould material according to theinvention contains a basic material which is present in a grain form, orcomparable particle form, and which as such is pourable, which, duringthe heating which inevitably arises during the casting, showsconsiderably less thermal expansion in relation to conventionally usedquartz sand.

[0024] The basic mould material accordingly guarantees, even with lowmaterial thicknesses, high dimensional stability in the manufacture ofcomplex shaped castings. The basic material, which is pourable in theunbound state, is mixed with a binding agent which has a differentexpansion behaviour on heating from the basic material. Because of thedifferent thermal expansion between the basic mould material and thebinding agent, the application of heat from the casting heat leads tothe release of the binding agent from the grains of the basic mouldmaterial. As a result, the binding agent accordingly bursts the mouldpart, when it expands more strongly than the basic material, in such away that the mould part loses its solid shape and can be easily removedout of and from the casting. Conversely, the expansion behaviour of thebasic mould material can be established in such a way that the change ofvolume which is inherent in the heating process breaks the binding withthe binding agent and the basic material is again pourable. An essentialfactor is that, inherently with the heating, the core or mould componentbreaks up, so that, after the casting has cooled, it has disintegratedinto loose individual parts which are easily removable.

[0025] By adjusting the thermal expansion behaviour of the inventivemould material according to the invention to the thermal expansionbehaviour of the molten metal which is to be cast, and by, at thesometime manufacturing this moulding material on the basis of a pourablebasic material it is attained that, after the cooling of the casting,the mould part which is at least in part surrounded by the casting or isadjoing by the casting, is broken into loose individual parts because ofthe forces which occur in the course of the cooling, which individualparts can then be easily removed. The breaking of the casting mouldparts is in this case effected as a result of the forces, caused bay thedifferent expansion of the cast metal and the mould material.

[0026] The invention has a particularly favourable effect with thecasting of components made from molten aluminium. Aluminium exhibits ahigh coefficient of thermal expansion, with the result that, in thecourse of the casting and solidifying of the melt, sufficiently highforces are exerted on the mould parts in contact with the cast part thatthe mould part concerned will be certain to break into smallerfragments. This proves to be particularly favourable if the mould partis a core mould.

[0027] A further property which is favourable for the invention of themould material constituted and used according to the invention is thatthe binding agent and the basic mould material are adjusted to oneanother in such a way that, with the casting mould parts produced fromthe mould material, the particles of the basic material are thermallystable and are not elastically bonded by the binding agent. The castingmould part which is produced from a basic mould material constituted inthis manner behaves throughout the entire temperature interval runthrough during the casting of the molten metal in a brittle manner, as aresult of which the disintegration of the mould parts wanted accordingto the invention is promoted.

[0028] The binding agent of the mould is for preference selected in sucha way that it does not disintegrate due to the effect of heat. In thisway the situation is avoided in which volumes are released in the corewhich could lead to a compliance of the mould concerned, which isundesirable according to the invention.

[0029] A further advantageous embodiment of the invention is that theparticles of the basic mould material exhibit an essentially round,spherical shape. The spherical shape of the basic mould material and theprevalence of point contacts between the basic moulding materialparticles, associated with this promotes the self-acting disintegrationof the mould parts as a consequence of the mechanical forces which ariseduring the casting and solidification of the melt. A basic mouldmaterial which fulfils this requirement particularly well issynthetically manufactured mullite. Accordingly, a further advantageousembodiment of the invention makes provision for the basic mould materialto contain at least a part of aluminium oxide sand (mullite), forpreference substituting for more than 50% or more than 70% of the quartzsand. Mullite exhibits a round grain shape and a density comparable toquartz sand. Accordingly, the mould materials manufactured from mulliteare considerably easier to process than, for example, known ZrO₂ sands.In addition to the advantages with regard to the mechanically-induceddisintegration of the mould parts, which is striven for according to theinvention, the round spherical shape of the mullite particles leads inpractice to simplified processability of the mould materialsmanufactured from such basic mould materials, and therefore inherentlyto a reduced wear on the tools and machines used for the manufacture ofthe mould parts. In addition to this, because of its low thermalexpansion a mould material with high mullite content has highdimensional stability during the manufacture of complex shaped castingseven with low material thickness.

[0030] It has surprisingly been shown that the disintegration of themould parts manufactured from the mould material constituted accordingto the invention occurs self-actingly with a time delay in relation tothe casting of the molten metal to such an extent that it has nonegative effect any more on the quality of the castings, which at thistime are already sufficiently solidified.

[0031] Due to its particular properties, mould material composedaccording to the invention is particularly well-suited for themanufacture of casting cores. These can be removed after casting withoutthe risk of damage to the finished castings.

[0032] Thermically a basic mould material composed of a mullite-quartzsand mixture and a mould material manufactured from this tends to havean insulating effect. Accordingly, these substances can be used in atargeted manner for such casting applications, in which there is indeedheating to above the critical temperature for quartz sand, of 573° C.,at which the thermal conductivity of the mould parts produced from thematerials concerned, however, plays a subordinate role or, respectively,the thermal conductivity is to be deliberately restricted.

[0033] Practical experiments have shown that by admixing a sufficientquantity of mullite sand to a quartz sand, the problems of spontaneousgeometry changes can be preempted, which occur with the use solely ofquartz sand as a basic mould material for the manufacture of slimfiligree castings. Essential in this respect is that the proportion ofthe Al2SiO5 sand is sufficiently high in each case for the change inlength of the quartz to be compensated for, which otherwise occurs withquartz sand with heating above the critical temperature.

[0034] With mould material constituted according to the invention,binding agent and basic material are, in addition to this, forpreference adjusted to one another in such a way that a casting mouldpart produced from the mould material has a low thermal conductivity.This property has the effect that, after the casting of the moltenmetal, the temperature difference between the cast material and themould part remains large, so that the risk of premature disintegrationof the mould part induced by thermal or chemical means is reduced to aminimum.

[0035] In addition to this, the disintegration of the mould cores isenhanced by the fact that the constituents of the mould material arematched to one another in such a way that basic mould material andbinding agent expand differently when heated, with the result that thebindings between them break under the heating which occurs with thecasting of the melt.

[0036] The invention can be realised in a manner particularlywell-suited to practice in that a mould material is processed which isformed from a mixture of a basic substance which is present in a grainor comparable particle form, and as such is pourable, and an inorganicbinding agent.

[0037] The advantage of the use of inorganic binding agents lies intheir better environmental compatibility and the fact that the mouldparts manufactured with such binding agents can be returned without anyproblem into the mould materials cycle. In this connection, mouldmaterials have proved to be particularly well-suited which are mixedfrom a binding agent based on water glass and a basic mould materialcomposed according to the invention. An essential consideration in thissituation, however, is that the expansion behaviour of the componentsmixed with one another differs in a sufficient manner.

[0038] It is particularly advantageous in this connection if the basicmould material and binding agent expand differently. In this case, afterthe onset of the heat from the casting heat, the binding agent becomesdetached from the grains of the basic mould material. This causes thebinding agent, if it expands stronger than the basic substance, to burstthe mould part in such a way that it loses its solid shape anddisintegrates into fragments. These can be easily shaken out of or fromthe casting without the risk of mechanical damage. Essential forensuring that with this variant the self-acting disintegration of themould part takes place, striven for according to the invention, isconsequently the different thermal expansion of the basic mould materialand the binding agent, in such a way that, under the effect of thecasting heat, the binding agent chips off from the basic mould materialparticles or inherently breaks up, as a result of the thermal stresseswhich occur between the basic mould material and the binding agent. As aresult of this brittle break behaviour of the binding agent after thehardening of the mould part, the binding between the individualparticles of the basic mould material is broken up, and the mould partdisintegrates. The remaining loose mixture of basic mould material andbinding agent fragments is pourable and can be easily removed out of orfrom the casting.

[0039] With the method according to the invention, the casting mouldpart is also manufactured by way of a mould material mixture composed inaccordance with the invention being shot in a known manner into the corebox of a core moulding machine. The mould material is then hardened, forexample in the manner described in DE 196 32 293 A1, in that an lowpressure is imposed on the hollow mould of the core box, heated to atemperature of 100° C. to 160° C., and the core mould is heated over aperiod from 20 to 30 seconds by the core box.

[0040] During this period the casting mould part becomes so solid thatit can be removed from the core box and can be placed into a heatingdevice, such as a microwave furnace, arranged outside the core mouldingbox. In this heating device it is heated at an adequate thermal outputuntil a sufficient volume of water has been drawn from it for completehardening.

[0041] As an alternative or supplement to a microwave heating arrangedoutside the core moulding box, the removal of water can also be effectedby sufficient heating of the core box itself, or by hot air gassing.These measures can in each case be combined with heating applied outsidethe core box. It is likewise possible for the removal of water to beeffected by microwave heating taking effect directly on the core mouldwhen still located in the core box.

[0042] If the mould part is subjected to heating outside the core boxfor hardening, the respective mould part can be sprayed with bindingfluid in order to increase the core surface hardness. The mould partstreated in this way exhibit increased stability with likewise increasedabrasion resistance, so that they can be stored without any problem andmeet the highest demands on dimensional stability. This proves to beparticularly favourable with regard to optimised quality of the castingwhich is to be produced if a water glass binding agent is used.

[0043] The invention is explained hereinafter in greater detail, on thebasis of a drawing showing an embodiment. The only figure shows indiagrammatic form a camshaft core 1 of a casting mould, not furtherrepresented, for the casting of a cylinder head from an aluminium castalloy.

[0044] Two recesses 3, 4, are formed in the underside 2 of the camshaftcore 1, spaced at a distance from one another in the longitudinaldirection, by means of which in each case the shape of the pillow blocksof the cylinder head which is to be manufactured, intended for thebearing of the camshaft is determined. Extending into the recesses 3,4respectively is a branch 5,6 of an oil channel core 8, which with itsmain section 7 extends parallel to and at a distance from the camshaftcore 1. The length A of the branches 5,6 is many times larger than itsdiameter B.

[0045] Likewise, the length C of the main section 7 of the oil channelcore 8 is many times larger than its diameter D.

[0046] The oil channel core 8 has been manufactured in an inherentlyknown manner in a conventional shoot-moulding machine from a mouldmaterial according to the invention, manufactured by mixing a basicmould material consisting of a mullite sand and quartz sand with a waterglass binder. Due to the proportion of mullite sand, it is guaranteedthat the oil channel core 8 expands uniformly and consequently in amanner which can be distinctly predetermined, also under the heat risingin the course of the casting of the cylinder head to be manufactured, upto more than 573° C.

[0047] Fractures in the area of the branches 5,6, extensions of the mainsection 7 in the area between the branches 5,6, and curvatures in thearea of the free ends of the main section 7, such as are encountered inthe conventional manner based on oil channel cores manufactured frombasic mould materials containing pure quartz sand, are in this wayreliably avoided. By means of the use of a mould material composed inthe manner according to the invention, cylinder heads and comparablecastings, which exhibit thin channels extending over considerablelengths, can thus be reliably manufactured with high precision in largenumbers in light metal casting.

[0048] During the pouring of the molten metal, which for preference isan aluminium melt or another light metal melt, and during the time inwhich the metal of the casting is still capable of flowing, the castingcores 1,8, are only insignificantly deformed because of the features ofthe basic mould material and the binding agents being adjusted to oneanother in accordance with the invention. The low thermal expansion ofthe basic mould material thus supports the reliable process attainmentof the dimensional requirements of the casting.

[0049] After a solidification and cooling period, during which thecasting attains a solidity sufficient for further processing, thefragments into which the respective casting core 1,8, self-actinglydisintegrates, as a result of the effect of the casting heat and due tothe different thermal expansion behaviour of the basic mould materialand the binding agent, are emptied out of the casting and reprocessed.In the course of the solidification process and in the cooling phaseafter the complete solidification of the metal, the respective castingcore 1,8, is subjected to high mechanical stresses due to thesubstantially higher solid body contraction of the cast metal incomparison with the casting cores 1,8. Because of the brittlenon-elastic consistency of the casting cores 1,8, this leads to thecasting cores 1,8 bursting apart into nodule-like fragments. The volumeand hardness of these is so low that the cast parts can be cleared ofsand solely by the application of vibration energy, since all the oldcore sand is present already released from the casting. Hammer blowsapplied by a compressed air hammer, such as are still required with theprior art, are not required for the sand removal.

[0050] The reprocessing of the fragments of the casting core can includegentle breaking into grainy particles. The grainy particles thusobtained can then be subjected to a metal separation process and dustextraction, in order to produce the state required for their reuse. Thecasting mould parts recycled to grainy material are then again used asbasic material for mould material composed in accordance with theinvention.

[0051] If mould materials are used in the manner according to theinvention, consisting of basic mould material such as synthetic mullite,mixed with water glass binding agent, then no emissions worth mentioningare incurred during the manufacture of the casting mould parts. As aresult, it is possible to avoid the casting defects extensiveprecautionary measures for the suction extraction of gases and elaboratetool cleaning processes required repeatedly with conventional proceduresas a result of gas formation. Environmental burdens and stress onoperating personnel are thereby reduced to a minimum.

[0052] If mullite or a comparabl inert refractory material is used as abasic substance for the mould material system according to theinvention, then a further advantage of the invention is realised in thechemical resistance of the basic mould material in relation to bindingagents and melt. This property ensures that, with the method accordingto the invention, a casting is obtained of which the surface is entirelyfree of residual sand adherence after the emptying of the fragments ofthe mould cores and mould parts, without additional cleaning measures.

REFERENCE FIGURES

[0053]1 Camshaft core

[0054]2 Underside of the camshaft core 1

[0055]3,4 Recesses

[0056]5,6 Branches of the oil channel core 8

[0057]7 Main section 7 of the oil channel core 8

[0058]8 Oil channel core

[0059] A Length A of the branches 5,6

[0060] B Diameter B of the branches 5,6

[0061] C Length of the main section 7

[0062] D Diameter of the main section 7

1-17 (canceled).
 18. A method for the manufacture of castings from amolten metal, in particular a molten light metal, comprising thefollowing steps: Manufacture of a casting mould part from the mouldmaterial, which is mixed from a pourable basic mould material which isinert in relation to the molten metal, exhibiting a proportion ofsynthetic mullite, which consists of particles of which the shape isessentially spherical, and an inorganic binding agent, wherein thethermal expansion behaviour of basic mould material and binding agentare adjusted to one another in such a way that the coefficient ofthermal expansion of the molten metal lies above the coefficient ofthermal expansion of the casting mould part manufactured from the mouldmaterial, Assembling a casting mould making use of the casting mouldpart, Pouring of the molten metal into the casting mould to form acasting, Cooling of the casting over a solidification and coolingperiod, in which the casting mould part self-actingly disintegrates intofragments, Removal of the fragments of the casting mould part from orout of the casting, Reprocessing of the fragments of the mould materialto form a pourable basic mould material.
 19. The method according toclaim 18, wherein the manufacture of the casting mould part comprisesthe shooting of the mould material into a hollow mould formed in a corebox, the preliminary hardening of the mould material shot into the corebox to form the casting mould part by the application of heat, and thehardening of the casting mould part in a heating device arranged outsidethe core box.
 20. The method according to claim 19, wherein the heatingdevice comprises a microwave heating.
 21. The method according to claim19, wherein the casting mould part is sprayed with binding agent beforethe hardening.
 22. The method according to claim 18, wherein thereprocessing of the fragments of the mould material comprises thebreaking of the fragments, the metal separation, the individualisationof grains, and/or dust extraction.
 23. A mould material for themanufacture of casting mould parts for the casting of molten metal, inparticular of molten light metal, consisting of a mixture of a pourablebasic mould material, inert in relation to the molten metal exhibiting aproportion of synthetic mullite, which consists of particles of whichthe shape is essentially spherical, and an inorganic binding agent mixedwith the basic mould material, wherein the thermal expansion behaviourof the basic mould material and binding agent are adjusted to oneanother in such a way that the coefficient of thermal expansion of themolten metal in each case lies above the coefficient of thermalexpansion of the casting mould part manufactured from the mouldmaterial.
 24. The mould material according to claim 23, wherein thebinding agent expands differently when heated in comparison with thebasic mould material.
 25. The mould material according to claim 23,wherein the binding agent is stable under the effect of the castingheat.
 26. The mould material according to claim 23, wherein the bindingagent is a water glass binding agent.
 27. The mould material accordingto claim 23, wherein the basic mould material consists entirely ofmullite.
 28. The mould material according to claim 23, wherein themolten metal is an aluminium melt.
 29. The mould material according toclaim 24, wherein its thermal conductivity is lower than that of themetal which is to be cast.
 30. The method according to claim 18performed with a basic mould material formed in accordance with claim24.
 31. The method according to claim 30, wherein the casting mould partis manufactured from the mould material is a casting core.
 32. Themethod according to claim 31, wherein the length (A,C) of the castingmould part is many times larger than its diameter (B,D).